Illumination optical system in exposure apparatus

ABSTRACT

Disclosed is an illumination system for illuminating a surface by use of light from a light source, which includes an emission angle conserving optical unit effective to emit the light from the light source at a constant divergent angle, and a diffractive optical element for producing a desired light intensity distribution on a predetermined plane, wherein the diffractive optical element is disposed at or adjacent a position where light from the emission angle conserving optical unit is collected.

FIELD OF THE INVENTION AND RELATED ART

[0001] This invention relates to an illumination system and, moreparticularly, to an illumination system suitably usable in anillumination optical system of an exposure apparatus for manufacture ofsemiconductor devices, which uses as a light source an excimer laser ofa vacuum ultraviolet wavelength region.

[0002] In a lithographic process among semiconductor devicemanufacturing processes, an exposure process for transferring, byexposure, a very fine pattern such as an electronic circuit patternformed on the surface of a mask onto a wafer is repeated plural times,whereby electronic circuits are produced on the wafer.

[0003] As regards the exposure method use in the exposure process, thereis a method in which a mask surface is held in contact to or in closeproximity to a wafer surface and, in this state, the mask surface isilluminated so that the pattern of the mask is transferred to the wafersurface. Also, there is a method in which a mask (reticle) placed at aposition optically conjugate with a wafer surface is illuminated, and apattern formed on the mask surface is transferred onto a wafer surface,by projection exposure, through a projection optical system. In anyexposure method, the image quality of a pattern transferred to a wafersurface is largely influenced by the performance of the illuminationsystem, for example, the uniformness of illuminance distribution on thesurface to be illuminated.

[0004]FIG. 10 is a schematic view of a general structure of aconventional illumination system, and it includes an inner typeintegrator and an amplitude division type integrator such as disclosedin Japanese Laid-Open Patent Application, Laid-Open No. 270312/1998

[0005] In FIG. 10, laser light emitted by a laser light source 101 isonce converged by a collimator lens 102 and then diverged, oralternatively, it is directly diverged by a negative lens, and the lightis incident on an inside reflection surface of an optical pipe 103 at apredetermined divergent angle.

[0006] The divergent laser light having a divergent angle passes throughthe inside of the optical pipe 103 while being reflected thereby, and aplurality of apparent light source images of the laser light source 101are produced on a plane (e.g., plane 110) perpendicular to the opticalaxis. Here, the laser beams, which appear as if they are emitted fromthe apparent light source images, are superposed one upon another on thelight exit surface 103′ of the optical pipe 103, such that a surfacelight source of uniform illuminance is produced at the light exitsurface 103′. The light beam from the light exit surface 103′ isdirected by way of a condenser lens 104, an aperture stop 105 and afield lens 106, to the reticle 107 surface. Since the light exit surface103′ of the optical pipe 103 is in an optically conjugate relation withthe reticle 107 surface, also the reticle 107 surface is illuminateduniformly.

[0007] If the shape of the optical pipe 103 is determined while takinginto account the length and width of the optical pipe 103 and thedivergent angle of the laser light provided by the collimator lens 102,the optical path differences of each laser beams emitted from theapparent light sources at the plane 110 toward each points on thereticle 107 surface can be more than the coherence length of the laserlight. This reduces the coherence with respect to time, and preventsspeckle on the reticle 17 surface.

[0008] However, in the structure of the illumination system shown inFIG. 10, a change in position of the light to be caused frequently as aresult of using a laser light source, may produce a variation inincidence angle of the light upon the surface being illuminated. Thisresults in non-uniformness of illuminance, upon the surface beingilluminated.

[0009] Further, in the illumination system of FIG. 10, multiplereflection is made at the inside reflection surface of the optical pipeso as to increase the number of apparent light source images, by whichuniform illuminance is attained. However, in order to assure theilluminance uniforming effect, the reflection times should be increasedand, therefore, the optical pipe should have a sufficient length. Since,however, in the vacuum ultraviolet region, absorption of light by theglass material, that is, a decrease of transmission factor, is large, ifan optical pipe of a length that assures sufficient illuminanceuniformness is used, the efficiency of light utilization may be degradedas a result of it.

SUMMARY OF THE INVENTION

[0010] It is accordingly an object of the present Invention to providean illumination system by which uniformness of a light intensitydistribution upon a surface to be illuminated can be maintained even iflight from a light source changes, and also by which the efficiency oflight utilization is improved.

[0011] In accordance with an aspect of the present invention, there isprovided an illumination system for illuminating a surface by use oflight from a light source, said illumination system comprising: anemission angle conserving optical unit effective to emit the light fromthe light source at a constant divergent angle; and a diffractiveoptical element for producing a desired light intensity distribution ona predetermined plane; wherein said diffractive optical element isdisposed at or adjacent a position where light from said emission angleconserving optical unit is collected.

[0012] In one preferred form of this aspect of the present invention,the illumination system may further comprise a multiple-beam producingelement, and a light projecting element for superposing light beams fromsaid multiple-beam producing element one upon another on the surface tobe illuminated, wherein the predetermined plane corresponds to a lightentrance surface of said multiple-beam producing element.

[0013] The illumination system may further comprise a zoom opticalsystem for projecting the light intensity distribution, produced by saiddiffractive optical element, upon the light entrance surface of saidmultiple-beam producing element at a predetermined magnification.

[0014] There may be a plurality of emission angle conserving opticalunits of different divergent angles, and wherein said emission angleconserving optical units are interchangeably set at a light path inaccordance with a change in magnification of said zoom optical system.

[0015] An emission angle conserving optical unit placed at the lightpath may be changed by another, whereby a numerical aperture of lightincident on the light entrance surface of said multiple-beam producingelement is substantially registered with a preset numerical aperture ofsaid multiple-beam producing means.

[0016] There may be a plurality of diffractive optical elements forproducing different light intensity distributions on the predeterminedplane, wherein said diffractive optical elements are interchangeably setat a light path to produce a desired light intensity distribution on thepredetermined plane.

[0017] The diffractive optical element may be a phase type or amplitudetype computer hologram.

[0018] The emission angle conserving optical unit may comprise a fly'seye lens having small lenses arrayed tow-dimensionally.

[0019] The emission angle conserving optical unit may comprise anaperture and a lens system.

[0020] In accordance with another aspect of the present invention, thereis provided an exposure apparatus, comprising: an illumination opticalsystem for illuminating a mask surface, as a surface to be illuminated,with use of light from a light source, said illumination optical systemincluding (i) an emission angle conserving optical unit effective toemit the light from the light source at a constant divergent angle, and(ii) a diffractive optical element for producing a desired lightintensity distribution on a predetermined plane, wherein saiddiffractive optical element is disposed at or adjacent a position wherelight from said emission angle conserving optical unit is collected; anda projection optical system for projecting a pattern formed on the masksurface, as illuminated with the light from said illumination opticalsystem, onto a wafer.

[0021] In accordance with a further aspect of the present invention,there is provided a device manufacturing method, comprising the stepsof: applying a photosensitive material to a wafer; illuminating a masksurface, as a surface to be illuminated, with use of light from anillumination optical system, wherein the illumination optical systemincludes (i) an emission angle conserving optical unit effective to emitthe light from the light source at a constant divergent angle, and (ii)a diffractive optical element for producing a desired light intensitydistribution on a predetermined plane, wherein the diffractive opticalelement is disposed at or adjacent a position where light from theemission angle conserving optical unit is collected; projecting, througha projection optical system, a pattern formed on the mask surface onto awafer; and developing the transferred pattern.

[0022] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic view of a general structure of anillumination system according to an embodiment of the present invention.

[0024]FIGS. 2A and 2B are schematic views, respectively, each forexplaining an emission angle conserving optical unit.

[0025]FIGS. 3A and 3B are schematic views, respectively, for explaininglight incident on a diffractive optical element.

[0026]FIGS. 4A and 4B are schematic views, respectively, for explaininga phase type CGH as an example of a diffractive optical element.

[0027]FIGS. 5A, 5B and 5C are schematic views, respectively, forexplaining examples of illuminance distributions to be produced by adiffractive optical element.

[0028]FIGS. 6A and 6B are schematic views, respectively, for explainingthe interchanging of emission angle conserving optical units.

[0029]FIG. 7 is a schematic view of a main portion of an exposureapparatus according to an embodiment of the present invention.

[0030]FIG. 8 is a flow chart for explaining semiconductor devicemanufacturing processes.

[0031]FIG. 9 is a flow chart for explaining details of a wafer processin the procedure of the flow chart of FIG. 8.

[0032]FIG. 10 is a schematic view of a general structure of aconventional illumination system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings.

[0034]FIG. 1 is a schematic view of an illumination system according toan embodiment of the present invention. Denoted in the drawing at 1 is alaser light source, and denoted at 2 is a light directing opticalsystem. Denoted at 3 is a beam shaping optical system, and denoted at 4and 6 are emission angle conserving optical units. Denoted at 5 is acondensing optical system, and denoted at 7 and 9 are relay opticalsystems. Denoted at 8 is an aperture stop, and denoted at 10 is adiffractive optical element. Denoted at 11 is a relay optical system,and denoted at 12 is an aperture. Denoted at 13 is a zoom opticalsystem, and denoted at 14 is a multiple-beam producing means. Denoted at15 is light projecting means including a condenser lens and the like.Denoted at 16 is a mark or reticle (surface to be illuminated) having acircuit pattern formed thereon.

[0035] The laser light source 1 may be any one of various excimer laserssuch as KrF, ArF, and F₂, for example. The diffractive optical element10 is a computer hologram designed to produce a desired illuminancedistribution (circular, ring-like or quadrupole shape, for example) atthe position of the aperture 12, through the relay optical system 11. Anamplitude type hologram, a phase distribution type hologram or aKinoform may be used for it The aperture 12 has a function for passingtherethrough only the illuminance distribution as produced by thediffractive optical element 10.

[0036] The multiple-beam producing means 14 comprises a fly's eye lenshaving a plurality of small lenses, or a fiber bundle, for example. Asurface light source defined by a plurality of point light sources isformed on the light exit surface of the multiple-beam producing means.The small lenses constituting the fly's eye lens may be provided by adiffractive optical element or, alternatively, a micro-lens array. Inthis embodiment, multiple-beam producing means 14 refers to an opticalelement having plural optical axes and plural regions of finite areasaround these optical axes, respectively, wherein, in each region, onelight flux can be specified.

[0037] The laser beam emitted from the laser light source 1 is directedby way of the light directing optical system 2, including a mirror and arelay lens (not shown), and it enters the beam shaping optical system 3which comprises a cylindrical lens and a mirror, for example. The beamsectional shape of the laser beam is transformed into a desired shape,by this beam shaping optical system 3. Then, the light enters theemission angle conserving optical unit 4.

[0038] Here, as shown in FIG. 2A, the emission angle conserving opticalunit 4 comprises an aperture 21 and a lens system 22. It has suchproperty that, even if the optical axis of the incident light shiftsslightly as depicted at 27 or 28 in the drawing, the emission angle 29 aof the light emitted therefrom is maintained constant. Alternatively, asshown in FIG. 2B, the emission angle conserving optical unit 4 may beprovided by a fly's eye lens having small lenses. In that occasion, theemission angle 29 b of the light is determined by the shape of the fly'seye lens. With use of emission angle conserving optical unit comprisinga fly's eye lens such as shown in FIG. 2B, the emission angle 29 b ofthe emitted light can be held constant.

[0039] The light emitted from the emission angle conserving optical unit4 at a desired emission angle α is collected by the condenser opticalsystem 5, and it is directed to another emission angle conservingoptical unit 6. Here, the light exit surface of the optical unit 4 andthe light entrance surface of the optical unit 6 are placed in therelation of Fourier transform planes (the relation of an object planeand a pupil plane, or the relation of a pupil plane and an image plane),through the condenser optical system 5. Further, the emission angle α isheld fixed. Therefore, even if the position of the light beam from thelaser light source 1 shifts, the distribution of the light incident onthe light entrance surface of the optical unit 6 is held fixed at thesame position on the plane, constantly.

[0040] The emission angle conserving optical unit 6 has a similarstructure and a similar function as of those of the optical unit 4described above, and the emission angle β of the light emitted therefromis constant.

[0041] The light emitted from the optical unit 6 at a desired emissionangle β is collected by the relay optical systems 7 and 9, and it isdirected to the diffractive optical element 10. In this embodiment, thediffractive optical element 10 is disposed at or adjacent a positionwhich is optically conjugate with the light exit surface of the opticalunit 6. With this structure, even if the light from the light sourcechanges slightly, the incidence position and the divergent angle (orconvergent angle) of the light entering the diffractive optical element10 can always be controlled at desired values. As a result, the lightintensity distribution to be produced at the position of the aperture 12(to be described later) can always be maintained constant. In FIG. 1,the element is disposed at a position slightly deviated from theconvergence point P of the light rays emitted from arbitrary points onthe light exit surface of the optical unit 6 (i.e., adjacent a positionoptically conjugate with the light exit surface of the optical unit 6),and it is illuminated with incident light having a divergent angle(convergent angle) γ. This will be described in detail, with referenceto FIGS. 3A and 3B.

[0042]FIGS. 3A and 3B illustrate the state of incident light on thediffractive optical element 10. In these drawings, denoted at 31 is thesurface of a diffractive optical element having a very fine step-likesectional structure formed on a substrate such as quartz, for example.Denoted at 32 a or 32 b is a single light spot to be produced by lightfrom a single small lens, where the optical unit 6 comprises a fly's eyelens and where small lenses constituting the fly's eye lens have ahoneycomb structure. Namely, light beams provided by a large number oflight spots 32 a or 32 b provide a light flux impinging on thediffractive optical element 10. Here, the width D of the light flux inFIG. 3A or 3B corresponds to the width D defined in FIG. 1 by brokenlines intersecting the diffractive optical element 10.

[0043] The size of each light spot 32 a or 32 b differs with therelative distance between the diffractive optical element 10 and theconvergence point P (i.e., deviation from the conjugate position of thelight exit surface of the optical unit 6). By making this relativedistance large, as shown in FIG. 3B, the size of the light spot 32 b maybe made large, such that the light spots are superposed one upon anotheron the diffractive optical element surface 31. By disposing thediffractive optical element 10 adjacent the conjugate position of thelight exit surface of the optical unit 6, as shown in FIG. 3B, damage ofthe element due to the energy concentration upon the diffractive opticalelement surface 31 can be prevented. The distance from the conjugateposition of the light exit surface of the optical unit 6 whereat thediffractive optical element 10 is disposed, may preferably be kept sothat a portion of the light spots 32 b comes out of the diffractiveoptical element 10 surface.

[0044] Referring to FIGS. 4A and 4B, the diffractive optical element 10will be explained. In this embodiment, a phase type computer hologram(CGH: Computer Generated Hologram) is used as the diffractive opticalelement 10. A computer hologram is a hologram which can be produced bycalculating an interference fringe pattern to be formed by interferencebetween object light and reference light and by directly outputting itby use of a pattern forming machine. An interference fringe shape toobtain a desired illuminance distribution as reproduced light can beeasily determined by optimization based on repetition of calculationsmade by use of a computer. FIG. 4A is a front view of a phase type CGHproduced in this manner, FIG. 4B schematically shows a sectional viewalong an arrow in FIG. 4A. In FIG. 4A, the phase distribution isillustrated by a tone distribution such as at 41. If the section isformed with a step-like shape such as depicted at 42 in FIG. 4B, thesemiconductor device manufacturing technology can be applied to themanufacture of the same. Thus, a step-like structure having a very finepitch can be produced relatively easily.

[0045] As regards the term “desired illuminance distribution” to beproduced by the diffractive optical element 10, it may be a circularilluminance distribution (FIG. 5A), a ring-like illuminance distribution(FIG. 5B), or an illuminance distribution called a quadrupole (FIG. 5C),all being suitable in accordance with the exposure condition used. Theilluminance distribution defined by the diffractive optical element 10is projected onto the light entrance surface of the multiple-beamproducing optical system 14, by means of the zoom optical system 13 (tobe described later). Thus, the above-described arrangement providesmodified or deformed illumination (oblique incidence illumination) meansin an illumination system of a semiconductor exposure apparatus, whichaccomplishes improvements in the resolution performance. Further, aplurality of diffractive optical elements may be mounted oninterchanging means such as a turret (not shown), so that they can beused interchangeably to change the illumination condition.

[0046] Referring back to FIG. 1, the light beam incident on thediffractive optical element 10 is amplitude-modulated or phase-modulatedas calculated, and it is diffracted thereby. The light goes through therelay optical system 11, and a desired illuminance distribution 12 a (asof FIG. 5A, 5B or 5C) having substantially uniform intensity within thedistribution is produced at the position of the aperture 12.

[0047] Here, the diffractive optical element 10 and the aperture 12 areplaced in the relation of Fourier transform planes. Based on thisrelationship, light diverged from an arbitrary single point on thediffractive optical element 10 contributes to the while illuminancedistribution 12 a. Namely, in FIGS. 3A and 3B, by an arbitrary lightbeam which forms the light spot 32 a or 32 b, regardless of itsirradiation position, an illuminance distribution 12 a (as in FIGS.5A-5C) suitable for the illumination can be produced at the aperture 12position.

[0048] Here, in the illuminance distribution 12 a, since the lightincident on the diffractive optical element 10 (CGH) has an extensionangle γ, here occurs a small blur corresponding to this angle. Thediffractive optical element 10 may be designed, as a matter of course,to produce a desired illuminance distribution while taking into accountsuch blur, beforehand.

[0049] Next, description will be made on the magnification change of thezoom optical system 13. A desired illuminance distribution 12 a beingsubstantially uniform within the distribution and being formed by thediffractive optical element 10, is projected by the zoom optical system13 onto the light entrance surface of the multiple-beam producingoptical system 14 at a desired magnification, as a uniform light sourceimage 14 a. Here, the term “desired magnification” refers to amagnification which functions to set the size of the uniform lightsource image so that the incidence angle ζ of the projected light on thesurface 16 to be illuminated has a value best suited to the exposure.

[0050] If, in respect to a desired magnification m, the light entranceside NA (numerical aperture) of the zoom optical system 13 as determinedby the angle δ is NA′, and the light exit side NA (numerical aperture)of the zoom optical system as determined by the angle ε is NA″, thefollowing relation is obtained:

NA′=m·NA″  (1)

[0051] Here, as regards the magnitude of the angle ε, from thestandpoint of illumination efficiency, preferably it should be close asmuch as possible to but not exceeding the NA with which the light canenter the multiple-beam producing means 14. Therefore, an optimum anglebeing dependent upon the the multiple-beam producing means 14 is set.Thus, as seen from equation (1), once an optimum magnification or theexposure process in a certain condition is determined, the optimum angleof δ is also determined.

[0052] In this embodiment, the value of angle δ depends on the size ofthe width D (FIG. 3A or 3B) of the irradiation region of the light beingincident on the diffractive optical element 10, and the magnitudethereof is dependent upon the emission angle α from the optical unit 4.On the basis of this, the emission angle conserving optical unit 4 isinterchanged in accordance with the illumination condition, to changethe width D of the irradiation region of the light impinging on thediffractive optical element 10. This will be described later in detail,with reference to FIG. 6.

[0053] When a uniform light source image is projected on the lightentrance surface of the multiple-beam producing means 14 in the mannerdescribed above, the illuminance distribution on the light entrancesurface is directly transferred to the light exit surface thereof. Then,light beams emitted from small regions of the multiple-beam producingmeans 14 are projected by the projecting means 15 onto the surface 16,to be illuminated, while being superposed one upon another. By this, thesurface 16 can be illuminated with a generally uniform illuminancedistribution.

[0054] Referring to FIGS. 6A and 6B, interchanging control of theemission angle conserving optical unit 4 will be described. In thesedrawings, denoted at 4 a is an emission angle conserving optical unithaving a smaller emission angle αa. Denoted at 4 b is another emissionangle conserving optical unit having a larger emission angle αb. Theremaining elements are similar to those having been described withreference to FIG. 1.

[0055] Generally, in an illumination system to be used in anillumination unit of a semiconductor device manufacturing exposureapparatus, it is required that the incidence angle of light to beprojected on the surface to be illuminated can be set as desired. Inthis embodiment, a plurality of emission angle conserving optical unitssuch as at 4 a and 4 b in FIG. 6 are mounted on a switching means suchas a turret (not shown), and these optical units are interchangeablyused in accordance with the requirement. With this arrangement, adesired incidence angle can be set to the light to be incident on thesurface to be illuminated.

[0056]FIG. 6A corresponds to a case wherein the incidence angle ζa ofthe light incident on the surface 16 is relatively small (this beingcalled as small σ value). In this embodiment, in order to make the σvalue small, the image 14 a of the illuminance distribution to beproduced by the diffractive optical element 10 on the light entrancesurface of the multiple-beam producing means 14 should be imaged at asmall magnification. Although this can be accomplished by changing themagnification of the zoom optical system 13, as described hereinbeforethe value ζa is set at an optimum angle dependent upon the entrance sideNA of the multiple-beam producing means 14.

[0057] Therefore, as seen from equation (1), once the magnification forattaining a desired σ value is determined, the divergent angle δa of thelight of the illuminance distribution, produced by the diffractiveoptical element 10 at the position of the aperture 12, is determinedfixedly. While the angle δa is determined by the width Da of lightincident on the diffractive optical element 10, this width is dependentupon the width 6 a of light impinging on the optical unit 6. Thus, theemission angle conserving optical unit is changed to the unit 4 a to seta smaller emission angle αa, thereby to narrow the light flux width 6 a.With this procedure, illumination of high efficiency and with a smallangle ζa (small σ value) is accomplished.

[0058] On the other hand, FIG. 6B shows an example where the σ value islarge. In this case, the emission angle conserving optical unit ischanged to the unit 4 b having a larger emission angle, to set the largeemission angle αb. By this, the width Db of light incident on thediffractive optical element 10 is made large, and the angle δb of lightdiverged from the illuminance distribution as produced by thediffractive optical element 10 is made large. Even though the image 14 bthereof is projected on the multiple-beam producing means 14 at a largemagnification, from the relation of equation (1), the angle εb can beset substantially the same as the above-described angel εa. With thisprocedure, illumination of high efficiency and with a large angle ζb(large σ value) is accomplished.

[0059] As regards the divergent angle of the light diverged from thediffractive optical element 10, since the angle γa (FIG. 6A) and theangle γb (FIG. 6B) are equal to each other, the divergent angle ofdiverged light is the same and, thus, the size of the illuminancedistribution 12 a at the aperture 12 position is unchanged even if theemission angle conserving optical units are interchanged.

[0060] As a matter of course, in order to obtain a desired illuminancedistribution in response to the σ value changing, if necessary, thediffractive optical element 10 may be interchanged by using a switchingmeans such as a turret (not shown), simultaneously in response to theinterchange of the optical units.

[0061] As has been described with reference to FIG. 2B, for example,even if the light from the laser light source 1 changes slightly due toany external disturbance, the emission angle of the light from theemission angle conserving optical unit 4 is conserved. Therefore, inFIG. 1, the position of the incident light on the optical unit 6 isunchanged. Further, since the emission angle of light from the opticalunit 6 is also conserved, there occurs substantially no change in theposition of the incident light on the diffractive optical element 10 orin the width of the light there. Thus, when the whole light source imageinside the small lenses of the multiple-beam producing means 14 isviewed macroscopically, it can be said that there is substantially nochange occurred. Consequently, the influence to the illuminancedistribution on the surface 9 to be illuminated becomes very small sothat it can be disregarded. This clearly suggests the advantages of thepresent invention that the system is very stable against a change oflight from the laser light source.

[0062] In accordance with embodiments of illumination systems accordingto the present invention, described hereinbefore, the followingadvantageous results are obtained.

[0063] (a) The incidence angle of light to the surface to be illuminatedcan be set at a desired value, and non-uniformness of illuminance can bereduced while attaining high-efficiency uniform illumination.

[0064] (b) Even if there occurs a change in the light in dependence uponthe laser light source, the incidence angle of the light incident on thesurface to be illuminated is stable. Thus, the adverse effect of thechange to the exposure can be removed.

[0065] (c) Use of a diffractive optical element having a small glassmaterial thickness in place of an optical pipe, ensures an illuminationsystem having a high efficiency even in a vacuum ultraviolet region inwhich the transmission factor is low.

[0066] For these reasons, an illumination system suitably usable,particularly, in an illumination optical system of a semiconductordevice manufacturing exposure apparatus, is provided.

[0067] Next, description will be made on an embodiment in which anillumination system of the present invention is applied to anillumination optical system of an exposure apparatus for manufacture ofsemiconductor devices such as semiconductor chips (e.g., IC or LSI),liquid crystal panels, CCDs, thin film magnetic heads, ormicro-machines, for example.

[0068]FIG. 7 is a schematic view of a general structure of asemiconductor device manufacturing exposure apparatus.

[0069] Denoted in the drawing at 71 is a beam shaping optical system fortransforming light from a laser light source 1 into a desired beamshape. Denoted at 72 is an incoherency transforming optical system fortransforming a coherent laser beam into incoherent light. Denoted at 73is a projection optical system, and denoted at 74 is a photo sensitivesubstrate such as a wafer, for example, having a photosensitive materialapplied thereto. Like numerals as of those of FIG. 1 are assigned tocorresponding elements, and description therefor is omitted.

[0070] The light emitted from the laser light source 1 is directed byway of a light directing optical system 2, and it enters the beamshaping optical system 71. The beam shaping optical system 71 comprisesa plurality of cylindrical lenses or a beam expander, and it functionsto transform the aspect ratio (longitudinal to lateral ratio) of thesectional shape of the light beam into a desired value.

[0071] The light thus shaped into a desired sectional shape by the beamshaping optical system 71 enters the incoherency transforming opticalsystem 72 for preventing interference of light upon the wafer 74 surfaceand production of speckle thereon, whereby the light is transformed intoincoherent light.

[0072] As regards the incoherency transforming optical system 72, anoptical system such as disclosed in Japanese Laid-Open PatentApplication, Laid-Open No. 215930/1991 may be used. In this opticalsystem, the incident light is divided by a light dividing surface intoat least two light beams (e.g., P-polarized light and S-polarized light)and, after an optical path length longer than the coherence length ofthe light is assigned to one of the divided light beams by use of anoptical member, these light beams are directed again to the lightdividing surface where one light beam is superposed on the other. Byusing a folding system such as above, mutually incoherent light beamsare produced.

[0073] The incoherent light produced by the incoherency transformingoptical system 72 enters the emission angle conserving optical unit 4,and it emits therefrom at a desired emission angle α. The light emittedfrom the optical unit 4 is collected by a condenser optical system 5 andis directed to the emission angle conserving optical unit 6. Here, thelight exit surface of the optical unit 4 and the light entrance surfaceof the optical unit 6 are placed in the relation of Fourier transformplanes through the condenser optical system 5. Further, the emissionangle α is held fixed. Therefore, even if the optical axis of the lightfrom the laser light source 1 shifts, the distribution of the lightincident on the light entrance surface of the optical unit 6 is heldfixed at the same position on the plane, constantly.

[0074] In the embodiment of FIG. 1, the light emitted from the opticalunit 6 is collected by relay optical systems 7 and 9 and is directed tothe diffractive optical element 10. In this embodiment, however, theserelay optical systems are omitted, and the light is directly directed tothe diffractive optical element 10. The diffractive optical element 10is disposed at a position slightly deviated from the position of thelight convergent point P′.

[0075] In this case, the optical unit 6 may be a fly's eye lens such asshown in FIG. 6B. Alternatively, to such structure, small lenses of adiffractive optical element disposed in array may be added. For the σvalue changing as has been described with reference to FIGS. 6A and 6B,the optical unit 6 and the diffractive optical element 10 may becombined into an integral unit, and this may be interchanged.

[0076] Then, with the procedure as has been described with reference toFIG. 1, light beams emitted from the small regions of the multiple-beamproducing means 14 are projected by the projecting means onto thesurface 16 while being superposed one upon another. Thus, the surface 16is illuminated with a generally uniform illuminance distribution. Then,the light which bears information about the circuit pattern, forexample, formed on the surface 16 is projected and imaged by theprojection optical system 73 onto the photosensitive substrate 74, at amagnification best suited to the exposure, whereby the circuit patternis transferred there.

[0077] The photosensitive substrate 74 is held fixed by vacuumattraction, for example, to a photosensitive substrate stage (notshown). It can be moved in translation upwardly/downwardly andforwardly/backwardly in the sheet of the drawing. The motion thereof iscontrolled by means of a measuring device such as a laser interferometer(not shown), for example.

[0078] Although in this embodiment the surface 16 is illuminated with agenerally uniform illuminance distribution, the emission angle of lightemitted from each small region of the multiple-beam producing means 14may be so set that different angles are defined with respect to twoorthogonal directions. In that occasion, the surface 16 can beilluminated with a slit-like shape.

[0079] Next, an embodiment of a semiconductor device manufacturingmethod which uses an exposure apparatus such as shown in FIG. 7, will beexplained.

[0080]FIG. 8 is a flow chart of procedure for manufacture ofmicrodevices such as semicondctor chips (e g. ICs or LSIs), liquidcrystal panels, CCDs, thin film magnetic heads or micro-machines, forexample.

[0081] Step 1 is a design process for designing a circuit of asemiconductor device Step 2 is a process for making a mask on the basisof the circuit pattern design. Step 3 is a process for preparing a waferby using a material such as silicon. Step 4 is a wafer process (called apre-process) wherein, by using the so prepared mask and wafer, circuitsare practically formed on the wafer through lithography. Step 5subsequent to this is an assembling step (called a post-process) whereinthe wafer having been processed by step 4 is formed into semiconductorchips. This step includes an assembling (dicing and bonding) process anda packaging (chip sealing) process. Step 6 is an inspection step whereinoperation check, durability check and so on for the semiconductordevices provided by step 5, are carried out. With these processes,semiconductor e vices are completed and they are shipped (step 7).

[0082]FIG. 9 is a flow chart showing details of the wafer process.

[0083] Step 11 is an oxidation process for oxidizing the surface of awafer. Step 12 is a CVD process for forming an insulating film on thewafer surface. Step 13 is an electrode forming process for formingelectrodes upon the wafer by vapor deposition. Step 14 is an ionimplanting process for implanting ions to the wafer. Step 15 is a resistprocess for applying a resist (photosensitive material) to the wafer.Step 16 is an exposure process for printing, by exposure, the circuitpattern of the mask on the wafer through the exposure apparatusdescribed above. Step 17 is a developing process for developing theexposed wafer. Step 18 is an etching process for removing portions otherthan the developed resist image. Step 19 is a resist separation processfor separating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are superposedly formed on the wafer.

[0084] With these processes, high density microdevices can bemanufactured.

[0085] In accordance with embodiments of the present invention asdescribed hereinbefore, even if the light from a light source changes,the illumination system can hold the uniformness of a light intensitydistribution on the surface illuminated. Also, the light utilizationefficiency is improved simultaneously.

[0086] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An illumination system for illuminating a surfaceby use of light from a light source, said illumination systemcomprising: an emission angle conserving optical unit effective to emitthe light from the light source at a constant divergent angle: and adiffractive optical element for producing a desired light intensitydistribution on a predetermined plane; wherein said diffractive opticalelement is disposed at or adjacent a position where light from saidemission angle conserving optical unit is collected.
 2. An illuminationsystem according to claim 1 , further comprising a multiple-beamproducing element, and a light projecting element for superposing lightbeams from said multiple-beam producing element one upon another on thesurface to be illuminated, wherein the predetermined plane correspondsto a light entrance surface of said multiple-beam producing element. 3.An illumination system according to claim 2 , further comprising a zoomoptical system for projecting the light intensity distribution, producedby said diffractive optical element, upon the light entrance surface ofsaid multiple-beam producing element at a predetermined magnification.4. An illumination system according to claim 3 , wherein there are aplurality of emission angle conserving optical units of differentdivergent angles, and wherein said emission angle conserving opticalunits are interchangeably set at a light path in accordance with achange in magnification of said zoom optical system.
 5. An illuminationsystem according to claim 4 , wherein an emission angle conservingoptical unit placed at the light path is changed by another, whereby anumerical aperture of light incident on the light entrance surface ofsaid multiple-beam producing element is substantially registered with apreset numerical aperture of said multiple-beam producing means.
 6. Anillumination system according to claim 1 , wherein there are a pluralityof diffractive optical elements for producing different light intensitydistributions on the predetermined plane, wherein said diffractiveoptical elements are interchangeably set at a light path to produce adesired light intensity distribution on the predetermined plane.
 7. Anillumination system according to claim 1 , wherein said diffractiveoptical element is a phase type or amplitude type computer hologram. 8.An illumination system according to claim 1 , wherein said emissionangle conserving optical unit comprises a fly's eye lens having smalllenses arrayed tow-dimensionally.
 9. An illumination system according toclaim 1 , wherein said emission angle conserving optical unit comprisesan aperture and a lens system.
 10. An exposure apparatus, comprising: anillumination optical system for illuminating a mask surface, as asurface to be illuminated, with use of light from a light source, saidillumination optical system including (i) an emission angle conservingoptical unit effective to emit the light from the light source at aconstant divergent angle, and (ii) a diffractive optical element forproducing a desired light intensity distribution on a predeterminedplane, wherein said diffractive optical element is disposed at oradjacent a position where light from said emission angle conservingoptical unit is collected; and a projection optical system forprojecting a pattern formed on the mask surface, as illuminated with thelight from said illumination optical system, onto a wafer.
 11. A devicemanufacturing method, comprising the steps of: applying a photosensitivematerial to a wafer; illuminating a mask surface, as a surface to beilluminated, with use of light from an illumination optical system,wherein the illumination optical system includes (i) an emission angleconserving optical unit effective to emit the light from the lightsource at a constant divergent angle, and (ii) a diffractive opticalelement for producing a desired light intensity distribution on apredetermined plane. wherein the diffractive optical element is disposedat or adjacent a position where light from the emission angle conservingoptical unit is collected; projecting, through a projection opticalsystem, a pattern formed on the mask surface onto a wafer; anddeveloping the transferred pattern.